1. Field of the Invention
The present invention relates to a tunable dispersion compensator and a method for tunable dispersion compensation for compensating dispersion of an optical signal.
2. Description of the Related Art
With the growing demand for information communication, there is a demand for greater transmission capacities. To meet this demand, efforts are being made to increase a transmission rate in an optical communication system. As the transmission rate is increased, a light waveform deteriorates due to chromatic dispersion in an optical fiber. The chromatic dispersion refers to a phenomenon that the rate at which an optical signal propagates through the optical fiber varies depending on the wavelength. Thus, a dispersion compensating fiber (DCF) is widely used to suppress the deterioration of the light waveform due to chromatic dispersion.
The DCF is designed to have a core diameter smaller than that of an ordinary optical fiber. This makes chromatic dispersion negative. This causes the chromatic dispersion of the DCF to have a characteristic having directionality opposite to that of the ordinary optical fiber. Therefore, combining the ordinary optical fiber with this DCF makes it possible to cancel chromatic dispersion produced by the ordinary optical fiber with the DCF and to suppress the deterioration of the light waveform.
However, the DCF does not have so great an amount of negative dispersion per length. Thus, the length of the DCF required to obtain an amount of dispersion compensation capable of long-distance communication is several tens of km. This causes a problem that the size of the equipment carrying out compensation of chromatic dispersion increases. Furthermore, since the DCF has quite a large transmission loss of an optical signal, an expensive light amplifier is required to compensate for the transmission loss. Furthermore, since the DCF has a small core diameter, the optical signal is trapped in a narrow area. This causes nonlinear interaction, deteriorating the light waveform. There is another problem that the amount of dispersion compensation cannot be made variable. Thus, the use of the DCF involves many problems. Therefore, there is a demand for realization of a tunable dispersion compensator, which solves these problems.
A first proposal for this is a tunable dispersion compensator using a ring resonator formed of a PLC (planar lightwave circuit) (see Japanese Patent Application Laid-Open No. 6-224860, for example). Here, the PLC refers to a silica glass optical waveguide formed by depositing silica glass, etc., on a silicon or silica substrate. The PLC also includes an optical component or optical circuit with an optical fiber array connected to the silica glass optical waveguide. This first proposal is constructed of a one-stage ring resonator placed between optical fibers or between an optical fiber and optical multiplexed receiving apparatus, wherein an optical multiplexed signal is output with a time delay. This first proposal is an attempt to compensate for all wavelengths to be used at once for chromatic dispersion which the optical multiplexed signal receives inside the optical fiber.
FIG. 1 shows an example of a ring resonator as a conventionally proposed tunable dispersion compensation device. This ring resonator 101 is constructed of a waveguide 102 which inputs and outputs an optical signal, an optical coupler 103 which distributes the optical signal and a ring-shaped waveguide 104 which delays the distributed optical signal. The ring-shaped waveguide 104 is provided with a heater 105 for heating control. The optical coupler 103 is made up of, for example, a directional coupler.
The operation of the ring resonator 101 shown in FIG. 1 will be explained. When an optical signal 106 is input from an optical fiber, etc., to the waveguide 102, the optical signal 106 is distributed to the ring-shaped waveguide 104 by the optical coupler 103 at a predetermined rate. The distributed optical signal is delayed by the ring-shaped waveguide 104. This delayed optical signal is returned by the optical coupler 103 to the waveguide 102 and output as an optical signal 107.
Here, when a current is passed to the heater 105 in the ring resonator 101, the temperature of the ring-shaped waveguide 104 is controlled by the heater 105. This changes the resonating central wavelength λ of the ring-shaped waveguide 104. That is, the ring resonator 101 changes this central wavelength through energization control by the heater 105.
FIG. 2 shows a variation caused by a temperature characteristic of a resonating central wavelength of the ring resonator in FIG. 1. The horizontal axis represents wavelength λ and the vertical axis represents delay time τ. The solid lines show that the heater 105 shown in FIG. 1 is controlled to a predetermined temperature. The central wavelength of the delay time spectrum at this time is λA. When the temperature of the heater 105 is increased in this condition, the central wavelength λA moves toward the long wavelength side as shown by an arrow 121. On the other hand, when the temperature is reduced, the central wavelength λA moves toward the short wavelength side as shown by an arrow 122.
Here, ring resonators may be connected in multi-stages (see Japanese Patent Application Laid-Open No. 2000-151513, for example). When the ring resonators are connected in multi-stages in this way, the optical signal 107 output from the 1st stage ring resonator 101 shown in FIG. 1 is input to a ring resonator in the next stage (not shown) connected to the waveguide 102. FIG. 3 shows a tunable dispersion compensator made up of such ring resonators connected in multi-stages. This tunable dispersion compensator 141 connects first to fourth ring resonators 1011 to 1014 in series along one common waveguide 102. In this FIG. 3, the same components as those in FIG. 1 are assigned the same reference numerals and explanations thereof are omitted as appropriate. Here, reference numerals denoting the components of the first to fourth ring resonators 1011 to 1014 are accompanied by subscripts “1” to “4” indicating the ring resonators 1011 to 1014 respectively.
In the tunable dispersion compensator 141 shown in FIG. 3, the heaters 1051 to 1054 of the first to fourth ring resonators 1011 to 1014 are controlled individually. In this way, the tunable dispersion compensator 141 can obtain an arbitrary dispersion slope. That is, as explained with FIG. 1 and FIG. 2, when only one ring resonator 101 is used, the delay time spectrum becomes a quasi-Gaussian distribution as shown in FIG. 2. However, shifting the respective resonating central wavelengths of the first to fourth ring resonators 1011 to 1014 and adding them together can form a linear slope.
FIG. 4 shows an example where a linear slope is formed. In this example, the heaters 1051 to 1053 of the first to third ring resonators 1011 to 1013 shown in FIG. 3 are controlled to a predetermined temperature and the resonating central wavelengths are set to λ1, λ2 and λ3. At this time, the central wavelength of the combined waveform the first to third ring resonators 1011 to 1013 is λA. When the heater 1054 is set to a predetermined temperature, the central wavelength of the fourth ring resonator 1014 is λB (=λ4) Here, the central wavelength λA and central wavelength λB are overlapped with each other. Thus, a dispersion slope 145 as indicated by a solid line in FIG. 4 is obtained.
Here, this dispersion slope 145 is contrasted with the inclination at the right of the signal waveform (delay time spectrum) resonated by a single ring resonator shown in FIG. 2. As a result, the dispersion slope 145 has a gentler slope. Furthermore, compared to the inclination at the right of the central wavelength of one ring resonator shown in FIG. 2, a quasi-straight line is obtained. Therefore, dispersion compensation can be performed using this dispersion slope 145.
However, with the tunable dispersion compensator 141 made up of the ring resonators 101 connected in multi-stages, high-order modes are derived from the fundamental mode when light transmits through the waveguide 102. Because of this influence, the S/N ratio (signal to noise ratio) of a main signal deteriorates, making it difficult to accurately determine central wavelengths of the respective ring resonators. As a result, it is difficult to correct chromatic dispersion of the optical fiber optimally.